1. Field of the Invention
The present invention relates to a method and apparatus for producing molded plastic products applicable to an optical scanning system such as laser beam printer, facsimile device, etc., an optical instrument such as video camera, an optical disc, and in particular, a molding method and a metallic mold for producing molded plastic products capable of transferring a high-accuracy mirror surface or a delicate concave/convex pattern to the molded product.
2. Discussion of the Background
Generally, as to an injection type molding method, the temperature of a metallic mold is made approximately equal to the thermal deformation temperature of the molded resin. Fused resin is then injected into a cavity of constant capacity formed in the metallic mold in order to fill the cavity with the resin. After gradually cooling the fused resin under a controlled pressure, the metallic mold is opened and the molded plastic products is removed therefrom.
However, according to the above-mentioned method, the temperature distribution at the time of cooling the resin may become the pressure distribution in some cases. For example in the case of producing thick structures and structures with unbalanced (uneven) thicknesses, although the molding cycle can be shortened, remaining pressure is produced in the thickness portion thereof and high-precision molded plastic products cannot be obtained due to shrinkage on the thick structure portion. Such matters are defects in conventional apparatuses and methods to be solved.
An injection type molding method that is less subject to such troublesome defects, for instance, is discussed in Japanese Laid-open Patent Publication No. 62-11619/1987 (hereinafter, called "first prior art"). The first prior art document describes that after the temperature of the metallic mold is set to a value equal to or higher than the softening temperature of the resin to be molded (glass critical point temperature of the non-crystal resin), the fused resin is injected into the cavity of constant capacity (volume) in order to fill the cavity with the resin, for making a gate seal. After producing a constant inner pressure of the resin, the fused resin is cooled, and the metallic mold is opened and the cooled resin is removed at a temperature not higher than the thermal deformation temperature. As such, the resin is cooled after the inner pressure of the resin is produced, and so the molded plastic products having the unbalanced thickness structure or thick structure, and large opening diameter can be molded with high accuracy.
There is another method described in Japanese Laid-open Patent Publication No. 61-19327/1986 (hereinafter, called "second prior art"). The second prior art document describes that, after the temperature of the metallic mold is set to a value equal to or higher than the softening temperature of the resin to be molded (glass critical point temperature of the non-crystal resin), the fused resin is injected into the cavity in order to fill the cavity with the resin.
Next, after the resin in the cavity is indirectly compressed and cooled at the same time through the molding piece for creating the cavity (hereinafter, called "cavity piece") by use of a pressurizing cylinder, the metallic mold is opened at a temperature not higher than the thermal deformation temperature and the cooled resin is removed therefrom.
In products molded according to such conventional processes, the resin is compressed at the time of cooling, so that even the molded plastic product of unbalanced thickness structure, thick structure, and large opening diameter can be molded with high accuracy.
In the molding method of the second prior art, since the temperature difference of the metallic mold occurred inevitably at the time of injecting/filling of the fused resin, and at the time of removing the molded plastic products, the molding cycle of the molded plastic products takes a long time to complete, and thus is problem to be solved.
Another method is described in Japanese Laid-open Patent Publication No. 63-114614 (hereinafter, called "third prior art"). In the third prior art, after setting the temperature of the metallic mold to a constant value that is approximately equal to the thermal deformation temperature, and the fused resin is injected into the cavity with a low pressure in order to fill the cavity with the resin, the fused resin is compressed from one side by use of a compressing mechanism in the metallic mold and thereby the contraction due to the cooling and solidifying of the resin can be compensated. In such way, molded plastic products of high accuracy can be obtained.
Still another method is described in Japanese Laid-open Patent Publication No. 8-234005/1996 (hereinafter, called "fourth prior art"). This fourth prior art document describes a method for manufacturing an optical reflection mirror, in which the filling of the resin is ceased immediately before the completion of a resin filling operation, and the resin is brought into close contact with the mirror forming surface. A wetting property (adhesion force) is lowered at the opposing surface and thereby the close adhesion force with the resin is also reduced, as is the amount of shrinkage on the surface with the close-adhesion force.
Another method is described in Japanese Laid-open Patent Publication No. 6-98642/1994 (hereinafter, called "fifth prior art"), directed to a method of manufacturing an optical reflection mirror. This document describes that the wetting property (adhesion force) of the surface opposing the mirror forming surface is lowered to a value lower than that of the mirror forming surface, and further the molded plastic products can be obtained without applying any controlled pressure to the plastic material that fills the metallic mold.
There in still another method as described in Japanese Laid-open Patent Publication No. 6-304973/1994 (hereinafter' called "sixth prior art"). This document describes that the pressure difference is produced after the injecting/filling operation between the transferring section and the air vent outlet is performed so that the shrinkage can be produced on a notifying outlet portion.
However, in the molding method of the first prior art, since it was necessary to make the resin inner pressure equal to the atmospheric pressure when the metallic mold was opened, the resin inner pressure at the time of injecting and filling the fused resin had to be made considerably high. Therefore, a high-cost metallic mold capable of enduring the high pressure had to be prepared, and a temperature difference of the metallic mold inevitably occurred when injecting/filling the fused resin and when removing the molded plastic products. As a result, and as identified by the present inventor, the molding cycle of the molded plastic products is lengthy.
In the molding method of the third prior art, since the resin was compressed by a compressing force received from the one side, and the resin was partially solidified at the time of compression, the unbalanced distribution of pressure occurred in the compressing direction for the resin having unbalanced structure and thick structure. Consequently, producing molded plastic products of high accuracy could not be obtained, which the present inventor identified as a problem to be solved.
In the molding method of the fourth prior art, since the resin was compressed by receiving a compressing force from the one side, the resin was partially solidified at the time of compression and so the unbalanced distribution of pressure occurred in the compressing direction for the resin having unbalanced thickness structure and thick structure. Consequently, obtaining molded plastic products of high accuracy could not be obtained with this method and is a problem to be solved.
Furthermore, in the molding method of the forth prior art, although the method was applicable to resin of not a thick structure, such as a mirror or the resin having a surface opposing thereto which can be shrunk, since almost all of another side is not wetted for the resin of thick structure and unbalanced thickness structure, the thermal conductivity from the fused resin to the metallic mold turned out to be unsatisfactory. In addition, cooling time was excessive, and was not applicable to uses requiring a high-accuracy transferring property on the opposing surface, such as for a lens or the like.
While this molding method obtained the high-accuracy molded plastic products even for the resin of unbalanced thickness structure and thick structure, the molding cycle turned out to be long, and high-accuracy molded plastic products could not be obtained for the resin of unbalanced thickness structure and thick structure when using a short molding cycle method.
In the fifth prior art, if the method was not limited to the surface opposing the transferring surface, the transferring property of the transferring surface was lowered, so the method was not applicable to operations requiring the transferring property of the opposing surface such as for lens or the like. Since a controlled pressure was not applied thereto, even though the side surface excluding the transferring surface utilized as the standard surface, it is impossible to realize the accuracy of the standard surface. This is a problem to be solved.
In the molding method of the sixth prior art, the shrinkage did not occur until the resin inner pressure, due to the cooling of the resin after filling the cavity with the resin, became not larger than the atmospheric pressure. In the molded plastic products of thick structure and unbalanced thickness structure, the pressure at the resin of thick structure portion rapidly became equal to or lower than the atmospheric pressure. However, regarding the timing thereof, since the resin in the thin structure portion had been already cooled and solidified under the initial high pressure, an internal distortion remained in the resin of thin structure, and so an accuracy of the transferring surface turned out to be lower due to the remaining pressure.
A conventional prior-art method of producing the molded plastic products is described hereinafter on the basis of FIG. 1 and FIG. 2. Hereupon, FIG. 1 is an explanatory diagram showing a conventional method of injecting and molding resin, and FIGS. 2a and 2b are explanatory diagrams showing general (prior-art) methods of molding the resin by injecting and compressing the same.
In FIG. 1, the reference numerals 1 and 2 respectively represent a pair of metallic molds that form therein an unbalanced thickness structure and thick structure cavity 3 of a constant capacity and having a transferring surface on at least one of the cavity surfaces. FIG. 1 shows a state of keeping the temperature of the metallic molds 1 and 2 to the thermal deformation temperature of the resin and filling the cavity 3 with the fused resin 4.
On this occasion, since the cavity 3 has the unbalanced thickness structure and the thick structure, the resin 4 is heated to the fusing temperature at the time of filling the cavity is sharply cooled from the tip end portion of the thin structure thereof. For this reason, as shown in FIG. 1, a shrinkage area A appears on the thick structure portion at the center portion thereof and the resin inner pressure remains on the tip end portion in the thin structure area at the same time. When the metallic mold is opened, the pressure is released and one resin expands. As the result, the molded plastic products of high accuracy cannot be obtained.
In FIGS. 2a and 2b, the reference numerals 5 and 6 respectively represent (a pair of) metallic molds forming therein an unbalanced thickness structure and thick structure cavity 7 of a constant capacity and having a transferring surface on at least one of the cavity surfaces. The above-mentioned cavity pieces 5 and 6 are respectively provided slidably of the upper metallic mold (die) 8 and the lower metallic mold (die) 9.
On this occasion, as shown in FIG. 2a, keeping the temperature of the metallic molds 8 and 9 approximately equal to the thermal deformation temperature of the resin, the cavity 7 is filled with the fused resin. Because the cavity of unbalanced thickness structure and thick structure is used, the resin 10 put in the state of fusing temperature at the time of filling the cavity starts to be sharply cooled and solidified from the both tip ends of the thin structure portion. Furthermore, in accordance with the solidifying of the resin, the cavity piece 6 is slidably moved and thereby the resin is compressed, as shown in FIG. 2b. Consequently, although the extent of shrinkage can be reduced, so as to avoid the problem illustrated in FIG. 1, the resin rapidly solidifies at the tip end portion B and so the solidified resin receives a compressive force.
For this reason, the resin inner pressure remains at the tip end portion B of the thin structure, and the pressure of the resin is released when the metallic molds 8 and 9 are opened, and further the resin expands at the same time. Therefore, the molded plastic products cannot be obtained with high accuracy.